Improvement in use of photometric methods for measurement of birefringence

Visual or graphical determination of isochromatic order in photoelastic models is not sufficiently accurate for many photoelastic observations. This is especially true in the long-wavelength spectral region where fringe orders are low and photometric devices must be used for observation. In such cases, some method of fractional-fringe measurement is required. The Tardy and Sénarmont methods are often chosen for convenience, with preference given to the Sénarmont method because of smaller system error. These methods are based on detection of the analyzer angle at which the light transmitted through the given point in the model is minimum. The experimental problem is poorly conditioned, and the procedure is incompatible with nonideal light-sensing apparatus. The equations of the Sénarmont technique show that the intensity of radiation is an even function of the rotation of the analyzer from the position which produces minimum intensity. The indeterminacy in the measurement of minimum-intensity angle may be reduced by observing the angles at which the intensity reaches arbitrary values greater than minimum. The average of two such readings on both sides of the minimum-intensity angle provides an improved measurement of this angle. Best results are obtained if observations are made where the intensity reaches half its maximum value. For a signal-to-noise ratio of 1000, this procedure improves results by a factor of 16. Improvement is greater asS/N is increased.